MATERIALS MOVE ACROSS THE CELL’S MEMBRANE SOME MATERIALS MOVE BY DIFFUSION –Diffusion= the process by which molecules spread out, or move from areas of high concentration to areas of less concentration –Occurs because molecules are always in constant motion; cells use diffusion to carry out life functions SOME TRANSPORT REQUIRES ENERGY CELL SIZE AFFECTS TRANSPORT

DIFFUSION Occurs naturally as particles move from area of high concentration to area of lower concentration The greater the difference in concentrations, the quicker diffusion occurs. Diffusion is how materials move in photosynthesis & cellular respiration. Diffusion is a type of passive transport (materials move without using cell’s energy.) Polar substances (sugar/salts/amino acids) diffuse via protein channel (still passive- no energy)

OSMOSIS Diffusion of water through a membrane is OSMOSIS. If you forget to water a plant, the soil dries, the roots dry, and water leaves the plant cells by osmosis, so the cell shrinks and the plant “wilts”.

Some Transport Needs Energy ACTIVE TRANSPORT –Uses energy to move material through membrane –Marine iguanas that swim & feed in salty ocean which causes lots of salt in blood. –They have special glands using active transport to remove salt from blood & blow it out of their nostrils. –Similar to our kidneys filtering blood for us.

TRANSPORT REQUIRING ENERGY, CONTINUED ENDOCYTOSIS: large bits of material get captured in pocket of membrane Pocket breaks off and forms package that moves into cell. Cells use this to fight bacteria & viruses by absorbing them. EXOCYTOSIS: a membrane within the cell encloses material needed to be removed Package moves to cell membrane, joins with it, and expels material Cells use this to flush out waste or expel proteins & hormones made by the cell.

CELL SIZE AFFECTS TRANSPORT Most cells are very small; most human cells are about 50 micrometers (.05mm), while bacteria are only 3-5 micrometers. The amount of cell membrane limits the ability of a cell to transport materials into and out of a cell. As a cell gets larger, there is a time when its volume grows more than its surface area, so not enough resources can get into the cell to feed it. Thin, flat cells or long skinny cells (muscle & nerve) give lots of surface area for plenty of food and resources to get into the cell.